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A new theory for X-ray diffraction

Paul F. Fewster

Acta Crystallographica Section A Foundations and Advances, March 2014, International Union of Crystallography

DOI: 10.1107/s205327331400117x

What is it about?

The theory of X-ray diffraction from crystals has been established for over 100 years; although it is still used, it cannot account for some of the experimental data. An imperfect theory combined with measured data can lead to the wrong structural model. This new theory is more complete and includes the diffraction from crystals in all directions, explains the diffraction from powders, results in better fits to measured data and removes some of the complexity describing a structural model.

Why is it important?

The speckle around a Debye-Scherrer ring can be from crystals of the same size. The intensity measured in a diffraction pattern is a proportion of the total calculated from a structural model. The diffraction peaks from a single crystal can appear at the Bragg angle yet not satisfy Bragg’s Law, making orientation assignment problematic. Intensity maxima form at the Bragg and harmonic positions for a single atomic plane separation, e.g. contributing to the unexpected 222 reflection in diamond.